Of all the moving parts in the Trump era -- the changes in the American diplomatic and military profile, the challenges the new president is hurling at the judiciary, the tensions between the legislative and executive branches, the dramatic stylistic changes in the presidency, ranging from shiny ties to Twitter outbursts -- none may be in such furious motion as the two parties, the steady rocks of our political system for more than a century and a half.

Much of that is changing in the era of Donald J. Trump, who once was a Democrat and thus -- we will not use this metaphor promiscuously with this president -- might be considered a bridge figure, though it is also possible that he is no more than a detour sign. And of course those stereotypes have been undergoing changes for some time; Newt Gingrich isn't taciturn, isn't stoic, isn't deliberate and isn't always frugal, and he was perhaps the greatest transitional figure in modern Republican politics, maybe even more so than Ronald Reagan.

The truth is that Trump -- so formidable a figure in the United States today that he makes mixed metaphors unavoidable -- is a giant celestial body in the political universe, warping the orbits of the other planet. And chief among the planets adjusting to new orbits, or perhaps careering out of long-established orbits, are the two parties.

Today's Republicans are utterly unrecognizable, not only from the party of its current members' fathers, but also unrecognizable from the party of only a few months ago. Gone, for example, is the watchword of George H.W. Bush, who made "prudent" a principle. Mr. Trump is less prudent than impulsive, which may be why he is president today and Jeb Bush, prudence personified, is not. Neither Bush would have hurled a brush-back pitch -- here goes another metaphor for today's collection -- at an Australian political leader, who would be more likely to be invited for clam rolls at Kennebunkport than dressed down like some misbehaving child at Kennebunk Beach.

These new Republicans are dancing to a new tune but don't yet know its rhythms. During the campaign, House Speaker Paul Ryan of Wisconsin performed a country reel with his party's nominee, dancing with him but never venturing too close. Now he's performing the Twist as kind of a congressional version of Chubby Checker (who now is 75 years old). Washington is accustomed to these sorts of things; remember how the Johnson girls, Lynda and Luci, taught the 36th president -- who favored the waltz and performed it on two separate occasions with Imelda Marcos -- how to dance the Watusi as the Great Society fell apart? For now, Ryan and his GOP dance partners are happy that the president is committed to their twin passions, repealing Obamacare and overhauling the tax code.

But the unspoken secret is that these Republicans are wary of the man at the podium, worried that he will be diverted from the orchestral score, horrified at the scat lines that emerge from deep within him, troubled that he might point his nuclear baton at Iran or North Korea, or maybe even Canada, whose prime minister's trip was delayed while his jetliner was de-iced.

(Now there -- de-icing -- is a metaphor that screams from beyond the 49th parallel. Indeed the journey of Justin Trudeau, who showed Mr. Trump that he had a deft hand at Twitter by welcoming to Canada the migrants refused by the United States -- was a mission of de-icing.)

The Democrats are no less confused. They lost their New Deal base to a billionaire in November with hardly a fight -- no nominee visits at all to Wisconsin (FDR won it three times), a curious obsession with rallies at universities whose audiences were congenial, even adoring, but very likely abstaining on Election Day. They're at a loss how to speak to their natural constituency. Their top figures are about Chubby Checker's age.

And yet the passion is on the left, with women marching, comedians mocking, speakers barking, lawmakers posturing, maybe even senators filibustering. It took Senate Majority Leader Mitch McConnell to transform Sen. Elizabeth Warren into a cult figure in this era of the figure of speech. She's now a legitimate presidential candidate, the sixth since 1960 from Massachusetts, half of whom won nominations.

The questions multiply for the Democrats, who likely will not answer them with anything approaching unanimity:
How do they square opposing the president when it means opposing the very people who until recently were the foundation of their party and the critical element of their constituency? Is there a danger in leaning too far left? Is there peril in being an opposition party without a smidgen of loyalty? (That didn't hurt the Republicans in the Obama years, but is that a precedent worth mimicking?)

That's without even considering Trump's nominee for the Supreme Court, Neil M. Gorsuch, who is the very portrait of a qualified high-court nominee even as he is the very personification of the judicial impulses the Democrats deplore. They dare not support him, nor dare oppose him. Mr. Gorsuch's declaration of independence in castigating the president for his snipes at the judiciary may have given Democrats cover, especially the 10 senators running for re-election next year in states Trump carried.

Do they cave and send him to the court for the next third of a century, or do they oppose him and fend off accusations that the Democrats have perfected the art of the Bork, a reference to their refusal to confirm Ronald Reagan's nomination of Robert Bork to the court? Do they extend the congressional paralysis, explaining their intransigence by arguing the Republicans wouldn't even conduct a hearing for Merrick B. Garland, President Obama's choice to replace Justice Antonin Scalia, who died a year ago this month?

They don't know the answer. But the author of this piece knows that his column is a mess of metaphors. He knows, too, that that itself is a metaphor.

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.